Indexing Service is no longer supported as of Windows XP and is unavailable for use as of Windows 8. Instead, use Windows Search for client side search and Microsoft Search Server Express for server side search.
The FilterFilesWithUnknownExtensions entry determines whether files with extensions that have not been registered will be filtered.
Set the value of the FilterFilesWithUnknownExtensions entry to zero if only registered file types are to be filtered. Set the value to one if unregistered file types are also to be filtered.